Pathetic service from MTN

I made the worst mistake of my life 3 years back by signing up for a laptop deal with MTN, I paid for that laptop for a full 36 month without skipping any payment and my contract reached its 36 month period two month ago, i have being receiving numerous calls from MTN to upgrade this device and i rejected the offer, i have been trying to call MTN for the retention department to cancel this contract and wow i stay on the line for more than 4 hours waiting for a consultant to cancel the correct but then if you call the upgrade department. it takes seconds to pick up and they have been sending me invoices every month. stay away from this pathetic service